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Re: Recent s.s.t.c work

Original poster: "K. C. Herrick" <kchdlh@xxxxxxx>



Tesla list wrote:
Original poster: Steve Ward <mailto:steve.ward@xxxxxxxxx><steve.ward@xxxxxxxxx>


Hi Ken,

[snipped]

>If anyone has some ideas as to how to maintain zero-current switching
>in a simple feedback-system, I'd like to hear them!  In the interim,
>I study the problem.


Im afriad i know of no simple way to completely cure this problem. What i suggest is tracking down where the majority of the delay is entering the loop. In fact, you dont even need to run any primary current for most of this testing. Just compare the input signal vs the output signal at various stages in your circuit. Perhaps compare the delay from feedback input, to gate driver input. Then compare from gate driver input to IGBT gate voltage. Perhaps there is something simple (like unwanted capacitance, or slow diodes) that is adding a lot of delay that could be avoided. I have basically given up on achieving real ZCS for my coils, but in your case, i would definately try to improve things if possible. Ideally you would get the phase lag down to about 20 degrees, which would mean you are hard switching about 1/3 of the peak current, max.
Yes, I'm going to start doing that tomorrow. But intuitively I feel that all one can do is minimize the delay--given the simple feedback scheme employed: the great big oscillator is just going to switch when it "wants" to, and that's that.

But perhaps that intuition is not so hot: I'm going to try some phase-shifting in the H-bridge simulation ckt I've just run; much easier than messing with the hdw!

I'm going to want to run some primary current so as to swamp out the pilot oscillator. That's no problem @ low input.

>Steve W., to answer your other questions:  1.  The max. input voltage
>will be ~twice the peak out of the variac, which is connected to go
>to ~140 V rms.  That yields ~390 V across the H-bridge.  I do
>incorporate a cycle-by-cycle over-current protection circuit that
>seems to work OK, but I have not set it yet (via a pot).


You are using 1200V devices, use that to your advantage! I would rather suggest applying 280VAC with a V-doubler. To keep the peak primary current in check, just use less drive cycles (4-16 cycles as i mentioned, though i only needed 7 cycles to generate the 12 foot sparks). This would be much better than running half the input voltage at twice the current. 

Good point; I'll have to look into that.


>2.  The feedback input signal is the output of my 2nd current
>transformer (following the 1 ohm resistor I've mentioned) clamped by
>4 diodes in series--parallel.  In other words, a ~2.8 V p-p square
>wave.  Paralleling those diodes is a 50 ohm resistor and that whole
>network refers to signal ground.


Question: why the extra 1:1 transformer? It is possibly adding un-wanted delay, check it out and see.

I will check that. The xtra xfmr is so I can bridge-rectify the voltage across the series 1 ohm resistor & then use that for over-current sensing, referring to logic ground. The 1:1 output is also referred to logic gnd but I can't so refer both at once w/out the xtra xfmr.


 > The capacitor of a
>Schmitt-trigger-gate oscillator connects to the "top" of that network
>rather than to ground, so that the oscillator provides my "pilot
>oscillator" signal with no spark but the feedback-signal takes over
>as soon as the primary starts to draw current, turning the oscillator
>into merely an amplifier for the duration of the spark event.  That
>seems to work seemlessly.


Could this capacitive coupling result in a phase shift of your feedback input? 
[snipped]

>
>Steve
Yes, I'll check that. But I think it is providing a lead, if anything, rather than a lag so perhaps there's room for tweaking there. Have to experiment with it.

KCH